The present invention relates to a variable-ratio transmission device, especially for motor vehicles.
So-called "automatic transmission" devices comprising an energy-transfer means with relative angular slip connected in series to a triple-access differential mechanism, are known.
The energy-transfer means generally consists of a hydraulic torque converter. It comprises, as a driving member, a hydraulic energy generator, called a pump, connected to the input of the transmission device and, as a driven member, a hydraulic motor, called a turbine, which produces mechanical energy from the hydraulic energy supplied by the pump and which is connected to one of the accesses of the differential mechanism.
The differential mechanism generally consists of a planetary gear train, of which an access other than that connected to the turbine is connected to the output of the transmission device. The third access of the planetary gear train can either be immobilised, to cause the planetary gear train to operate as a reducer, or be connected to one of the other two accesses, to cause the planetary gear train to operate by direct drive.
The torque converter has the advantage of replacing the friction clutch and of itself giving rise, as a function of its turbine/pump speed ratio, to some variable multiplication of the torque which makes it possible to produce the transmission with a reduced number of separate mechanical ratios, for example only two ratios, with a single planetary gear train, as stated above.
On the other hand, conventional automatic transmissions have some serious disadvantages. Their efficiency is very poor, especially in town traffic. Moreover, their weight is very great, their cost is high and they are of great complexity, especially when they comprise numerous gear ratios, this being the current trend.
Admittedly, the most recent transmissions comprise a direct-drive device which is put into operation when the vehicle reaches a cruising speed, in order at this stage to eliminate losses in the converter. Nevertheless, this device, which further increases the complexity, does not solve the problem of the very high losses caused by the converter at low speed, particularly in the acceleration phase.
Furthermore, U.S. Pat. No. 3,426,618 makes known a device, the input of which drives one of the accesses of the differential mechanism directly and drives another access of the differential mechanism by means of a torque converter and a reducer.
Thus, only some of the power passes via the converter and the losses in the converter can be reduced.
This results, however, in a relatively complex device which, in one of the examples given, comprises two epicyclic gear trains for providing only two transmission ratios. Moreover, the control of the change from one ratio to the other is complex, with the simultaneous control of a clutch and of a brake as a function of parameters, such as the rotational speed and/or the torque to be transmitted, which necessarily have to be supplied by measurement or detection devices. This system can bring about jolts or, on the contrary, momentary breaks in transmission.